Characterization of tantalum target materials for mass separation applications at CERN-MEDICIS facility

Tantalum-based materials are widely used as high-temperature target materials for radionuclide production and mass separation at ISOL-type facilities such as CERN-ISOLDE and CERN-MEDICIS due to their refractory nature and exceptionally high melting point. Their performance is strongly governed by microstructural stability under prolonged heating in high vacuum, which directly affects isotope diffusion and release. In this work, the thermal behavior and microstructural evolution of rolled tantalum foils, tantalum carbide (TaC) pellets, and tantalum carbide prepared with multi-walled carbon nanotubes (TaC-CNT) additive were systematically investigated under conditions relevant to both on-line and off-line operation. Samples were resistively heated in vacuum to temperatures up to 2000 °C for extended durations and characterized before and after heat treatment using scanning electron microscopy and complementary surface analysis techniques. Metallic tantalum foils exhibited significant microstructural evolution consistent with high-temperature diffusion processes, including surface restructuring and grain coarsening and solid-state sintering, leading to consolidation of stacked foils and the loss of open diffusion pathways. TaC pellets showed improved thermal stability but underwent significant grain coarsening and pore closure after high-temperature exposure, resulting in reduced porosity. In contrast, the TaC-CNT composite demonstrated a slightly finer grain size and, crucially, significant microstructural stability after reheating to high temperatures during operation at CERN-MEDICIS, which is considered a key parameter for porous ceramic samples under testing. These microstructural trends are consistent with operational observations at CERN-MEDICIS, where TaC-based targets show lower detectable terbium release-start temperatures than metallic tantalum foils, while TaC-CNT targets further reduce the release-start temperature and show improved reuse behaviour under the tested operating conditions. Unlike tantalum foils, porous target materials should retain relatively high porosity and resist pore collapse or further sintering upon reheating; the results for the TaC-CNT composite indicate a high potential for target reusability. Most importantly, the TaC-CNT composite strongly mitigated sintering-induced degradation. For conventional target materials, the mass-separation efficiency in a second use typically decreases by roughly an order of magnitude. In contrast, for the TaC-CNT target the efficiency was reduced by only a factor of about two between the first and second collections. This represents a major improvement in target reusability. The results highlight the critical role of microstructural control in optimizing high-temperature target materials for efficient and sustained radionuclide production.